Black/bruising behind the knee?

Last year these black lines started out on the bends of my knees in the back. My knees felt kinda achy, but not in a lot of pain. Over the next few days the small black lines got bigger.

After a few days the lines on my legs were massive bruises covering my back legs from the knee up to my underwear line This is when I started feeling dizzy and couldn't do simple tasks like stand up long enough to shower.

In the ER, they said it was clearly blunt force trauma and refused to even entertain theories as to how bruises like these would just appear cause they there was just no way.

They ran blood tests and found I was anemic, and a little dehydrated (there was a family crises going on at home so I hadn't been eating and drinking very much) and then they did an ultrasound and were shocked there was no tissue or blood vessel damage.

They sent me home, I kept getting weaker and the bruises got bigger and darker, I went back and they ran blood work on me and that's when real concern set in as my blood count was so low I now needed a blood transfusion.Two units had to be used.

After all that, the bruises began to lighten and the edges turn yellowish.To this day I have no answer for what happened to me. I tell doctors what happened and they just shrug their shoulders. I wouldn't care expect for sometimes I feel that same aching in my legs and sometimes there's a dark line. But it never gets as dark as it did a year ago and seems to go away within a day...but I'm a little scared.